What’s Wrong With My Trees?
Common abiotic disorders impacting landscapes
By Janet Hartin
With so much emphasis on pest identification and control in parks, it’s easy to forget that the majority of disorders impacting landscape trees result from abiotic (non-living) disorders. However, there are several techniques that greatly reduce the chances of damage from abiotic disorders:
• Selecting healthy plants that are well suited to the climate and microclimate
• Incorporating recommended planting techniques
• Providing a soil environment that optimizes a healthy root system
• Implementing sound cultural-management practices (irrigation, fertilization, pruning, aeration, etc.).
Keeping notes regarding the overall maintenance of a park or field, including planting and care practices that were used, can be very useful when trying to determine why a tree is not thriving. These field notes will help identify what changes in the plant-maintenance regime (e.g., irrigation, fertilizer, pruning), climate (temperature change, high winds, etc.), or construction activities may have compromised the health of the tree. Also, if a tree was planted too deeply, deep roots may remain dry while surface roots are subjected to crown rot. In other cases, weed cords or mowers damage the lower trunks of trees and provide an entryway for fungus.
In some cases, a wide array of abiotic and biotic symptoms may co-exist, making identification tricky. Outbreaks of insect pests, such as bark beetles, which do not invade healthy trees, can quickly infest stressed, water-deficient trees. Because drought-stressed trees cannot recover from damage as quickly as healthy trees, pathogens that cause several canker diseases are often common during and just after drought.
A good starting point to determine if the damage is the result of an abiotic disorder rather than due to a pathogen or insect is to look at the landscape as a whole. Is the decline impacting a single species or a wide array of species? Damage from many insects and diseases is often restricted to a single species (at least initially), whereas chemical toxicities from poor water quality or overfertilization tend to cause a general decline across several species subjected to the same treatment. Usually, uniform damage to multiple species limited to a single area of the landscape signifies one or more abiotic factors. In some cases, biotic injury may be obvious and abiotic disorders can be ruled out. Examples include tell-tale signs of diseases, such as discolored exudate dripping from cankers; mycelial strands; and mushrooms around the base of trees.
Let’s look at some of the major abiotic issues impacting landscape trees:
• Alkaline soils
• Excessive salt buildup
• Too little or too much water
• Soil compaction/poor aeration
• Heat stress
• Herbicide drift
• Air pollution
• Girdled/circled roots
• Mechanical injury
• Changes in soil grade.
Soil pH is a measure of soil acidity. A pH of 7 is neutral, a pH below 7 is acid, and a pH above 7 is alkaline. Measuring the level of pH when testing soil is very important since, in many cases, nutrients become “tied up” and unavailable in soils with too low or too high pH, so adding more fertilizer will not solve the problem. The pH of urban landscapes tends to be higher than corresponding native, undisturbed soils. Soil pH is an important chemical property because it affects the availability of nutrients to plants and the activity of microorganisms in the soil. A pH measurement is therefore a crucial part of a soil-testing program.
Alkaline Soils. Western soils tend to be alkaline, with average pH ranges from 7.1 to 7.6. Alkaline soils often result in nutrients such as iron, zinc, and manganese being tied up in the soil and unavailable for uptake by the plant even though no true deficiency exists. This underscores the importance of requesting a pH test in addition to the typical nutrient analysis. Once the pH is reduced, these important micronutrients can be taken up by the tree. Applying sulfur to soils directly or in a fertilizer such as ammonium sulfate can reduce alkalinity over time, remedying the symptoms. Adding gypsum, which contains sulfur, can also reduce alkalinity. Unfortunately, there is no remedy for calcareous soils and soils irrigated with water containing calcium carbonate. In addition to leaching, tolerant tree species should be selected.
Acidic Soils. In many states, acidic soils with pH levels below 7.0 are common. In these cases, adding pulverized limestone is advised to increase soil pH. Note that limestone varies in form. The most commonly applied form consists of calcium carbonate (calcitic limestone). While it works well to increase pH in many situations, the other form (dolomitic limestone) is a better choice for soils with low levels of magnesium. Avoid applying too much lime since an overapplication can result in reduced nutrient availability of iron, manganese, and zinc, as found in high pH soils, previously discussed.
Lime recommendations are based on the pH of the soil and a SMP buffer index, which determines how much lime, if any, is required. More lime is required under a low SMP index than a high index. In general, soil tests with a pH reading under 6.0 are routinely subjected to the SMP buffer index. More specific information on this topic and tables listing rates of lime to apply shown in both pounds per 1,000 square feet and in pounds per 100 square feet may be found here: http://www.extension.umn.edu/garden/landscaping/implement/soil_ph.html
Keep in mind that effects from lime applications take many months to visually correct and for optimal results, liming should take place to a depth of 6 inches prior to planting whenever possible. While lime can also be applied around established trees by mixing it into the soil a few inches (be careful to not disrupt roots), this method is generally not as effective as mixing lime into soils deeper prior to planting.
Excessive Salt Buildup
Damage due to excessive fertilizer application or water high in certain ions appears as marginal leaf browning or leaf scorch. With increasing numbers of landscapes being irrigated with recycled water, leaching as a routine maintenance component is necessary. Symptoms of fertilizer/salt toxicity due to excess foliar-applied nutrients usually conform to the portion of the foliage where the overexposure occurred.
Salt damage can result in injury, and even death, of sensitive plants. Soils often contain a wide assortment of water-soluble salts, many of which are necessary for plant growth and development. However, high concentrations of chlorides, sulfates, and nitrates of calcium, magnesium, sodium, and potassium can harm a wide variety of landscape plants. Because trees store food reserves in roots, they do not require nutrients immediately after transplanting. In general, woody plants benefit from a fertilizer nitrogen, phosphorus and potassium [N-P-K] rations of 3-1-2 or 4-1-2. Unless nitrogen symptoms occur, mature landscape trees should not be fertilized with more than .25 lb/actual N per inch of trunk circumference.
Soils containing excessive levels of exchangeable sodium are referred to as sodic or alkali soils. They contain a high concentration of sodium compared to calcium and magnesium, causing fine- and medium-textured soils to lose aggregated structure and become impervious to air and water. Sodic soils can also be toxic to sensitive plants. It is important to note that salts can be absorbed by foliage from ocean spray and sprinkler irrigation in addition to root uptake. Symptoms of root-absorbed toxicities include stunting and chlorosis initially, which can be followed by foliar necrosis and defoliation. Older leaf edges and tips are generally the most symptomatic. Another visual diagnostic tool is the presence of a white or black crust on the soil surface.
Salinity can be diagnosed by electrical conductivity (EC) tests of soil taken from the root zone and analyzed by a reputable laboratory. Remember that most salinity often occurs in the top portion of the soil, near the soil surface; do not take your sample from this level unless plants were recently seeded and/or transplanted. Instead, take soil samples from a deeper level of soil consistent with the rooting area.
Excessive applications of fertilizers can also result in salt toxicity. Fertilizer formulations also impact the level of toxicity. For instance, potassium chloride poses a higher salt hazard than potassium sulfate, as do fresh, undecomposed animal manures and sewage sludge.
Saline soils can be remedied by leaching salts below the root zone. In some cases, when the irrigation water is more saline than most plants will tolerate, less-sensitive species should be selected. No amount of leaching using the original source of saline water will correct a salt problem.
Too Little or Too Much Water
Inadequate water can adversely impact all plants, even drought-resistant species, until they are established. Significant wilting and dieback often occur in newly planted container plants when roots dry out too much between irrigations. If not corrected, plants will eventually die. Established plants that have moderate to high evapotranspiration rates may also suffer from a lack of water, resulting in leaf scorch and defoliation in addition to wilt. Over time, they succumb to stunted growth, significant stem and branch die-back (particularly in upper limbs), and eventual death.
Many established landscape plants suffer from too much water. As woody plants age, they prefer to be irrigated more deeply and less often than when they were initially planted and establishing their root systems. Low oxygen levels insufficient for plant growth and development occur in saturated, poorly drained soils such as clay loams.
Soil Compaction, Poor Aeration
Another common problem faced routinely by arborists and landscapers is compacted soils with poor structure and drainage. Soil compaction often results from heavy equipment and machinery sitting on top of the soil surface, and even heavy foot traffic over time. The condition worsens under soils with high moisture content. Compacted soils have low water infiltration and permeability rates, reducing pore space once filled with air. Compacted soils can severely restrict root growth of nearby trees.
Acute symptoms include wilting and leaf drop. Over time, leaf chlorosis and necrosis, cankers, additional leaf drop and canopy thinning occur and entire branches may die. Under anaerobic conditions, soil may smell like rotten eggs and darken in color.
Preventive measures include limiting construction traffic around landscape sites, as well as the addition of surface barriers. Foot traffic should be kept to a minimum, using defined pathways. It is imperative that trees be protected during nearby construction. Damage from heavy equipment may be irreversible. Consider using double, overlapping sheets of plywood to protect the soil surface if the use of heavy equipment is unavoidable. Additionally, a three-inch layer of wood-chip mulch can reduce soil compaction.
Heat stress often occurs when a tree is planted in a climate zone with higher summer temperatures than the tree is adapted to. Over time, high air and soil temperatures lead to irreversible damage. In some cases, trees well-adapted to an inland or desert climate can become stressed and die under temperatures they would normally endure if a water deficit develops.
The American Horticultural Society heat-zone map (based on the number of days with temperatures above 86 F) is available here: http://www.ahs.org/gardening-resources /gardening-maps/heat-zone-map. Many nurseries are now including this information on plant tags.
Keep in mind that heat islands created by dark surfaces such as asphalt parking lots can increase temperatures around tree plantings between 5 and 25 F. Even light-colored rock mulches can increase soil temperatures enough to cause damage.
When applied improperly, many pesticides result in serious damage to untargeted plants. This type of plant injury often occurs as the result of drift from herbicides containing 2,4-D applied on windy days. Twisted, curled foliage and distorted stems and leaf damage occur days to weeks later. New growth is especially sensitive. Immediately hosing down the foliage with water before the herbicide dries can reduce the damage. In other cases, damage occurs as a result of spraying wettable sulfur applications for the control of powdery-mildew when temperatures exceed 90 F.
Although air pollution has declined over the past two decades, due largely to enhanced vehicle emission standards, ozone and sulfur dioxide-sensitive trees continue to be impacted. Ozone damage appears in the form of mottling, chlorosis, spots, and bleaching of young leaves. Sensitive tree species include Ailanthus altissima (Tree-of-heaven), Cercis canadensis (Redbud), and Liquidambar styraciflua (Sweetgum). Symptoms of sulfur dioxide damage include bleached or tan necrotic areas between veins on young broadleaf leaves and brown tips on conifers (particularly pines).
Girdling, Circled Roots
Circled tree roots can lead to stressed trees that can topple under wind velocities that would not lead to failure otherwise. Circled roots can occur when trees are left too long in containers; trees are planted in too small of a planting hole; or trees are planted in an otherwise large enough hole (e.g., 2.5 times the width of the container) filled with compost. In the last case, roots will often prefer the higher-quality compost to the surrounding native soil and never expand horizontally. In some cases, more than one of the above conditions co-exists. Don’t accept nursery stock with circled roots and plant-container stock at the same depth, but in holes at least 2.5 times the width of the pot. Do not add soil amendments; gently notch the outside edge of the planting hole and return the removed soil back into the hole.
Mechanical injury to trees results in minor to major injury, sometimes resulting in death. Often, damage from tight, unremoved staking ties extends into the vascular system of the trunk, cutting off the flow of water and nutrients. Early symptoms include wilting, stunting, and general tree decline. Bark can also be damaged from mowers and weed trimmers. It is important to avoid trunk injury since damaged bark cannot be repaired.
Change in Soil Grade
Grade changes from construction activities should be kept several yards from trees. Even small changes in soil depth can result in major root system injury. As little as 4 to 6 inches of soil applied on top of a root zone of a mature tree can dramatically reduce the amount of oxygen available to tree roots and result in root mortality and sometimes tree death. Symptoms include stunting, slow/reduced tree growth and, eventually extensive dieback. Mature, established trees are much more at risk than younger or newly planted trees. Lowering the grade is not recommended as well because it can cause lower trunk and root injury and reduces the nutrient and moisture supply to the roots.
Costello, L.R, E.J. Perry, N.P. Matheny, J.M Henry, and P.M. Geisel. (2003). Abiotic Disorders of Landscape Plants: A Diagnostic Guide. University of California Agriculture and Natural Resources Publication 3420. Oakland, Calif., 242 pp.
Ducklow, Laura and Daniel Peterson. Modifying Soil and pH: Sustainable Urban Landscape Information Series (SULIS). University of Minnesota, referenced 3/14/2018 from: http://www.extension.umn.edu/garden/landscaping/implement/soil_ph.html
Hartin, J. S. (Winter 2017). ”Common Abiotic Disorders Impacting Landscape Trees.” UC IPM Green Bulletin, 7 (2), 3-5.
Hartin, J. S. et al. (2014). “Sustainable Landscaping in California.” UC ANR publication #8504. Oakland, Calif. 12 pp.
Moorman, Gary. (2012). Abiotic Diseases of Woody Ornamentals. Pennsylvania State University, College of Agricultural Sciences. Penn State Publication Distribution Office, 112 Ag. Admin. Bldg., University Park, Pa. 16802. 3 pp.
Schutzki, R.E. and B. Cregg. (2007). “Abiotic Plant Disorders; Symptoms, Signs and Solutions, A Diagnostic Guide to Problem Solving.” Michigan State University Extension Bulletin E-2996. 16 pp.
Janet Hartin is an Environmental Horticulture Advisor for the University of California’s Cooperative Extension, Division of Ag and Natural Resources for San Bernardino, Riverside, and Los Angeles Counties. Reach her at firstname.lastname@example.org.